<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">
        .main {
            width: 300px;
            height: 300px;
            margin: auto;
            margin-top: 50px;
            background: blue;
            /*border: 1px solid transparent;*/
        }

        .main p {
            width: 200px;
            height: 200px;
            margin-top: 30px;
            background: green;
        }
    </style>
</head>
<body>
<div class="main">
    <p></p>
</div>
</body>
</html>